Instrumentation ports.
This in-house experiment on the gust response of an annular turbine
cascade was particularly designed to provide data to compare with
the results of a typical, linearized gust-response analysis. Reduced
frequency, Mach number, and incidence were varied independently.
Except for the lowest reduced frequency, the gust velocity distribution
was nearly sinusoidal. For the high inlet-velocity series of tests,
the cascade was near choking. The mean flow was documented by
measuring blade surface pressures and the cascade exit flow, and
high-response pressure transducers were used to measure the unsteady
pressure distribution. Inlet-velocity components and turbulence
parameters were measured using hot wire anemometry. In addition
to the synchronous time-averaged pressure spectra, typical power
spectra are included for several representative conditions.
The gusts were generated by a rotor consisting of 3.17- or 4.76-mm-diameter
pins. Either 6, 12, or 24 pins were used, resulting in a gust
reduced frequency of 2.5, 5, or 10. The annular turbine cascade
had 24 blades, and it was positioned 3.9 axial chordlengths behind
the rotor. The figure above illustrates the positions of instrumentation
ports, and the following figure illustrates representative root-mean-square
(rms) spectra. The synchronous peaks could have been obtained
by linear phase-lock averaging; however, the rms-averaged spectra
also include the nonsynchronous origin and the random pressure
fluctuations. The frequency units are engine orders; thus, the
synchronous peaks appear at the frequency equal to the number
of rotor pins used to generate wakes. The position-axis units
correspond to the blade port numbers in the instrumentation figure; thus,
position 0 corresponds to the port nearest to the leading edge
on the suction surface side, and position 11 corresponds to the
port nearest to the leading edge on the pressure side.
The gust amplitude varied somewhat with the reduced frequency; however, it did not appear to have a dominant effect. Unsteady, synchronous-response blade pressures depend strongly on reduced frequency and incidence. Mach number dependence is weak for negative incidence and significant for positive incidence at lower reduced frequencies. The mean blade-pressure distribution depends, to some extent, on the reduced frequency, particularly for the negative incidence and the higher inlet Mach number. At a reduced frequency of 10, an inlet Mach number of 0.27, and a positive incidence, magnification of the turbulent pressure fluctuations on the suction side of the aft portion of the blade resulted in a significant excitation concentrated at an integral engine order much higher than the synchronous excitation frequency.
Previous articleLast updated May 5, 1997
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